1. Field of the Invention
The present invention relates to a flash memory incorporating microcomputer, and more partucilarly, to the testing of the microcomputer.
2. Description of the Related Art
Microcomputers have been broadly incorporated into cameras, audio-visual apparatus, automobiles and the like. Generally, a control program for operating a microcomputer is stored in a read-only memory (ROM). However, since the control program needs to be changed as occasion demands, the mask ROM has been replaced by an erasable and programmable ROM (EPROM), an electrically-erasable programmable ROM (EEPROM), or a flash EEPROM which is called a flash memory.
A prior art flash memory incorporating microcomputer includes input/output ports, a serial ccommunication interface, a mode control unit for setting various operation modes, a flash EEROM for storing a control program, a random access memory (RAM) for storing temporary data, a read-only memory (ROM) for storing a writing program for writing the control program into the flash EEPROM, a central processing unit (CPU) and internal buses for connecting these elements to each other (see JP-A-5-266219). This will be explained later in detail.
In the prior art microcomputer, although the CPU is tested in a CPU testing mode and the flash EEPROM is tested in a flash EEPROM testing mode, the flash EEPROM is not tested by the CPU. Therefore, even if the CPU testing mode and the flash EEPROM test mode both succeed, it is impossible to guarantee the operation of the flash EEPROM by the CPU.
Also, the ROM is active only for an on-board writing operation mode; in other words, the ROM is inactive for the other operation modes. Therefore, when the ROM is incorporated into one chip of the microcomputer, the chip area thereof is increased, thus reducing the manufacturing yield.
Further, since the serial communication interface is fixed to one of the types of communication systems, an upper system is limited.
Further, in order to write the control program into the flash EEPROM, since the control program is temporarily stored in the RAM or a buffer of the CPU, the time required for a writing operation of the control program is long.
Still further, when the frequency of a clock signal is changed by an upper system or the type of the serial communication interface, the time period of a writing operation and the time period of a flash erasing operation are also changed.